Depends on how difficult your frame is, how many bikes you have, and how often you replace cables.
If you have a real struggle getting it sorted then £45 is nothing, if it saves hassle and frustration. The more the kit is used, the cheaper it becomes.
I wasn't looking forward to doing by Bianchi in the Summer, as had never done it before. Toyed with buying the kit, but gave it a go first. I managed to get the cables into the frame fine, but getting them out of the letterbox exit from the frame was tricky on first go. Took a step back and made a simple deflector from a plastic lid from washing machine tablet box. Literally a strip narrow enough to fit the opening and just long enough to hold and reach the other side of the frame. Second attempt saw the cables emerge from said opening with ease.
Had this not worked after a dozen attempts, then I would have no doubt bought the kit without complaint. The right tools make the job simple.